176 REPORT—1850. 
The construction of this instrument, and the method employed, may perhaps be _ 
more easily understood by an examination of the annexed figures. 
Fig. 1 is a vertical section of a sirene intended to act on the air. A is a cylindrical 
brass box, about two inches in diameter and one inch in height, and having a closed 
top, B, about a quarter of an inch thick, with a perfectly plane upper surface highly 
polished. In the bottom of this box there is an opening for the introduction of the 
tube or “ porte-vente”’ C, by which wind is forced by bellows into the box. Openings, 
D, are pierced angularly through the top B, as also represented in the plan of the top 
and revolving disc, in fig. 2, and in the side elevation of the same parts, fig. 3. These 
holes—16 in number—are disposed in a ring, and are equidistant from each other. 
They are each of the same size, and the spaces between them are slightly larger than 
the holes themselves, in order that when the revolving disc E is in motion, its cor- 
responding ring of holes, G, may be quite cut off from any communication with the 
box A. 
The revolving disc E has a long boss upon its upper side for adjustment upon the 
vertical spindle F, which runs in a jewelled centre at the top in the upper part of the 
guide-frame, and at the bottom has an adjustable screw-centre in the box A. The 
holes G in this disc correspond in every respect with those in the top of the box A; 
they are pierced however at an opposite angle, in order that the pressure of the 
upward currents or jets from the box beneath may cause the disc to revolve*. The 
two surfaces of the disc and top of the box are highly polished, and are so adjusted 
as to cause no perceptible friction, and yet to work close enough to prevent the 
escape of air from between them. This is difficult to accomplish in practice, and 
the beauty and accuracy of the surfaces produced by the French makers of these 
instruments are very remarkable. 
When air is forced into the box A, the jets issuing from the holes D, impinge on 
the oblique surfaces of the corresponding holes in the disc E, and thus cause it to 
revolve at a rate proportioned to the pressure. 
Now, let it be supposed that one hole only was pierced in the box, and one in the 
disc, the air in its passage would force round the disc, and during its revolution the 
wind within the box would be prevented from passing to the air without the box, 
until the hole in the disc came round to the hole in the box, when the wind would 
rush through as before and thus keep up a regular rotatory motion, causing a stroke 
or pulsation on the atmosphere at each revolution. Ὁ 
When these pulsations are perfectly regular, and attain to a rapidity of thirty-two 
in a second, a musical note will be obtained appreciable to most ears, although a very 
grave note. As the pulsations become more rapid the notes become more acute. 
The greater the number of holes the more rapid the pulsations. 
Having concluded his explanation of this air-sirene, the Professor described the 
plan he had adopted in using it under water. In the diagram, A is a large square 
cistern, the sides of which are of strong plate glass, and the bottom of mahogany in 
avery strong frame. The sirene, B, made as already described, is firmly fastened 
down in the bottom of the cistern, a piece of caoutchouc being interposed between 
the surfaces to prevent the’ movement of the sirene from acting on the bottom of the 
cistern, - 
The details of the instrument are analogous to those of the air sirene, except that 
the holes in the box and disc are larger and only 8 in number. The actuating water 
is brought by the gutta-percha tube C, from a reservoir elevated about 30 feet 
above the cistern, through a hole in the bottom into the box B, governed by a stop- 
cock D. 
On turning the cock D, the water in the reservoir rushes into the cylindrical box, 
forces its way through the holes pierced in the upper part of the box, which strike 
those pierced in the disc, and thus keep up a rotatory motion. 
It must be observed, however, that unless the cistern be filled so as to com- 
pletely cover the sirene, the water forced through the revolving disc will strike the 
air; for although it is a jet of water that is cut off by the rotation of the disc till the 
hole in the dise comes round towards the hole in the box, yet nevertheless it is the 
* Strictly, the section, fig. 1, ought not to show any augularity in the holes, as the angle 
is in relation to the circumferential line of the disc, but they are so drawn to make the 
movement quite clear, 
